In order to still be failsafe after long transmission oil change intervals, in the extreme for so called service lift fillings the transmission valve has to have a high level of robustness. This high level of robustness can be achieved with large clearances for moving parts which, however, degrades control quality.
A hydraulic valve is known from DE 10 2011 053 023 A1 which has a high level of robustness in addition to a high level of control quality. The high level of robustness is achieved in that contaminant particles in the operating medium cannot lead to a seizing of the transmission valve since the armature can impart axial forces that are large enough for the transmission valve to break free under all circumstances. Simultaneously the hydraulic valve has a high level of control quality which is achieved through plural configurative measures. Thus, in particular the transversal forces between the armature and a pole tube are minimized.
A configurative measure for reducing the transversal forces is a very tight running tolerance between the armature and the pole tube which is achieved by a very thin separation layer instead of for example a sleeve or a thick coating. A very thin separation layer of this type is advantageously provided with a layer thickness of 10 μm to 60 μm. The thin separation layer can be achieved for example chemically or through a galvanic process. For example chemical nickel plating can be used for this chemical method. Thus, a layer thickness of 45 μm has proven ideal. The minimum thickness of 10 μm can be implemented at least theoretically. The methods currently used facilitates producing a layer thickness of 20 μm or greater. Contrary to for example galvanic nickel plating the chemical method does not use any electrical voltage that is applied through electrodes. The layer thickness achieved through chemical nickel plating is very homogenous.